1. Field of the Invention
This invention relates to a method and a system for shift control of an automatic transmission for a vehicle, and more particularly to improvements in a method and a system for shift control of an automatic transmission for a vehicle, wherein there are provided at least a first transmission section and a second transmission section, which are capable of shifting automatically and independently of each other, and wherein the first transmission section and the second transmission section are simultaneously and alternately shifted, to thereby achieve multi-gear shifts.
2. Discussion of Background
As the FF (Front engine--Front drive) has been used more widely in the driving system of vehicles in recent years, there has been an increased tendency toward high rpm and high output in an engines. An automatic transmission in a vehicle of the FF type presents a disadvantage in that it is extremely difficult to increase the number of plates in a multi-plate clutch and in a multi-plate brake of the transmission, both being frictionally engaging devices, due to the restriction placed upon the total length of the automatic transmission. In view of the above described disadvantage, to improve the durability of the frictionally engaging devices in the automatic transmission, there has been proposed an automatic transmission, wherein the number of gear stages may be increased by use of the existing automatic transmission added thereto with a so-called overdrive mechanism.
For example, in an automatic transmission, wherein an overdrive mechanism is added to an underdrive mechanism of three forward gear stages, the underdrive mechanism in the form of a first transmission section and the overdrive mechanism in the form of a second transmission mechanism may be simultaneously or alternately shifted as shown in FIG. 4, so that multi-gear shifts of six forward gear stages can be achieved.
With the above-described automatic transmission, wherein the first transmission section and the second transmission section are simultaneously or alternately shifted to achieve the multi-gear shifts, there are gear stages achieved due to the first transmission section and the second transmission section being shifted in directions opposite to each other. In this case, there has occurred a disadvantage that, unless the timing of these shifts into these gear stages is adequately and strictly controlled, fluctuations in torque are high, whereby an uncomfortable feeling is experienced by the driver.
More specifically, as shown in FIG. 4, when the automatic transmission as a whole is shifted up from 2nd gear stage to 3rd gear stage and from 4th gear stage to 5th gear stage for example, the first transmission section is shifted to high, whereas the second transmission section is shifted to low. When the automatic transmission as a whole is shifted down from 5th gear stage to 4th gear stage and from 3rd gear stage to 2nd gear stage, the first transmission section is shifted to low, whereas the second transmission section is shifted to high. For the above-described reasons, unless the shifts are effected accurately with adequate timing, the fluctuations in output torque can be high, in an extreme case, a sensation of a down shift performed immediately after an up shift or an up shift is performed immediately after a down shift, is felt.
On the other hand, to insure the durability of the frictionally engaging device of the transmission, there are instances where the normal order of shifting, i.e. 1st-3rd -5th-6th gear stage in FIG. 4 with the overdrive mechanism added thereto is changed to an order of 1st-3rd-4th-6th gear stage, for example. According to this method, the former and the latter examples are equal in terms of number of gear stages available, however, instead of the 3th-5th gear shift, which is performed by the frictionally engaging device (so-called direct clutch) where the durability is most severely tested, the overdrive mechanism (the second transmission section) is instead shifted, with 2nd gear stage of the first transmission section being retained to achieve 4th gear stage, while the 4th-6th gear shift is performed with the shift point being slightly low rpm's so that the durability of the frictionally engaging device can be enhanced. Normally, the above-described shift is performed only when a throttle opening (engine load) is high, whereas, for a low throttle opening, the order of shifting pattern of 1st-3rd-5th-6th gear stage is performed as in the past.
However, even in the above-described case, there occur gear stages which are achieved by shifting the first transmission section and the second transmission section in directions opposite each other as in the 4th-5th gear shift or the 5th-4th gear shift. As a consequence, the aforesaid disadvantages similarly can occur.